吉斯.如何在父类的字段中注入不同的对象

Question

我在Guice字段注入方面遇到问题。考虑以下类的层次结构。

abstract class Base{
@Inject
protected MyService myService;
}
class A extends Base{
@Inject
private AnotherService anotherService;
}
class B extends Base{
...
}

我希望在运行时有两个不同的MyService实例——一个注入A类的所有对象，另一个注入B类的对象。我知道如何通过构造函数注入实现这种行为：

bind(MyService.class).annotatedWith(Names.named("forA")).to(MyServiceImpl.class).in(Singleton.class);
bind(MyService.class).annotatedWith(Names.named("forB")).to(MyServiceImpl.class).in(Singleton.class);
class A extends Base{
@Inject @Named(value = "forA")
public A(MyService service1, AnotherService service2) {
this.myService = service1;
this.anotherService = service2;
}

问题是迁移到构造函数注入会非常复杂，所以我想坚持使用字段注入。有可能像我想要的那样调整场注入吗？

Answer 1

我会使用来自Guice的自定义注入来实现这一点。

首先，我将创建一个注释来注释我想要基于超类类型注入的MyService。

@BindingAnnotation
@interface MyServiceInject {
}

然后我会用注释来注释我的MyService字段。

static abstract class Base {
@MyServiceInject
protected MyService myService;
}

现在我们需要一个自定义的TypeListener，每当我们遇到注入点时都会调用它。

这就是我们将如何创建它。

static class ClassBasedMyServiceInjectionListener implements TypeListener {
@Override
public <I> void hear(TypeLiteral<I> typeLiteral, TypeEncounter<I> encounter) {
Class<?> clazz = typeLiteral.getRawType();
while (clazz != null) {
for (Field field : clazz.getDeclaredFields()) {
if (field.getType() == MyService.class && field.isAnnotationPresent(MyServiceInject.class)) { //if type of field is MyService and it has MyServiceInject annotation
encounter.register(new ClassBasedMyServiceInjector<>(field,
typeLiteral.getType(),
encounter.getProvider(MyServiceA.class),
encounter.getProvider(MyServiceB.class)
)
); //Now register a MemberInjector for this encounter.
}
}
clazz = clazz.getSuperclass();
}
}
}

现在我们需要MemberInjector，它看起来像这样。

static class ClassBasedMyServiceInjector<T> implements MembersInjector<T> {
private final Field field;
private final Type superClassType;
private final Provider<MyServiceA> myServiceAProvider;
private final Provider<MyServiceB> myServiceBProvider;
ClassBasedMyServiceInjector(Field field, Type superClassType, Provider<MyServiceA> myServiceAProvider, Provider<MyServiceB> myServiceBProvider) {
this.field = field;
this.superClassType = superClassType;
this.myServiceAProvider = myServiceAProvider;
this.myServiceBProvider = myServiceBProvider;
field.setAccessible(true);
}
public void injectMembers(T t) { //this will be called when guice wants to inject members
try {
if (superClassType == A.class) {//if super class is of type A
field.set(t, myServiceAProvider.get()); //inject MyServiceA
} else if (superClassType == B.class) { //if super class is of type B
field.set(t, myServiceBProvider.get()); //inject MyServiceB
}
} catch (IllegalAccessException e) {
throw new RuntimeException(e);
}
}
}

最后，我将在模块的配置方法中绑定我们的自定义TypeListener，如下所示。

@Override
protected void configure() {
bindListener(Matchers.any(), new ClassBasedMyServiceInjectionListener());
}

希望这能有所帮助。